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Sea Level Rise 13.2.2 Past Events Sea level rise is a dynamic phenomenon that is constantly evolving, the impacts of which are not associated or reported as singular events. It is already affecting Bay Area communities. In the last century, San Francisco Bay water levels have risen 8 inches. 13.2.3 Location San Mateo County is a peninsula county, meaning it is subject to two types of sea-level rise hazard exposures:  The eastern side of the County is exposed to the San Francisco Bay, which is more of a closed system.  The western side of the County is exposed to the Pacific Ocean and the more dynamic sea-level rise conditions associated with wave action. The inundation areas used for this assessment are a combination of scenarios from OCOF (6.6 feet of Pacific Ocean coastline sea-level rise by 2100, with 100-year storm) and the ART program (9 feet of San Francisco Bay coastline sea-level rise by 2100). Mapped inundation areas were aggregated for a singular sea-level rise assessment. Figure 13-1 shows the extent and location of these combined areas. 13.2.4 Frequency The probability of sea-level rise inundation in San Mateo County by 2100 is high. The sea-level rise projections for 2100 in the OCOF and ART program scenarios used for this assessment correlate to 0.98 to 1.35 inches per year over the next 80 years. Sea level rise projections are periodically revised as climate models are improved and updated with new data and observations. 13.2.5 Severity The severity of sea-level rise to the County of San Mateo will become greater over the next 30 to 80 years. The severity could be exacerbated by the following conditions:  Daily tidal inundation—As sea level rises, the amount of land and infrastructure subjected to daily inundation by high tides—also known as increases in mean higher high water—will increase. This would result in increased permanent future inundation of low-lying area.  Annual high tide inundation (King Tides)—King Tides are abnormally high, predictable astronomical tides that occur about twice per year. they are the highest tides that occur each year during the winter and summer when the Earth, moon and sun are aligned. Winter King Tides may be amplified by stormy weather, making them even more significant. King Tides result in temporary inundation associated with nuisance flooding, such as inundation of low-lying roads, boardwalks, and waterfront promenades.  Extreme high tide inundation (storm surge)—When Pacific Ocean storms coincide with high tides, storm surge can elevate Pacific Ocean and San Francisco Bay water levels and produce extreme high tides. Such storm surge events occurred on January 27, 1983, December 3, 1983, February 6, 1998, January 8, 2005, and December 31, 2006. Extreme high tides can cause severe inundation of low-lying roads, boardwalks, and promenades. They can exacerbate coastal and riverine flooding, cause upstream flooding, and interfere with stormwater outfalls. 13-3